Heat exchanger



' 19440 H. M. GRAHAM ET AL 2,341,319 t HEAT EXCHANGER Filed OG't. 31, 1941 Patented Feb. 8, 1944 UNITED STATES PATENT OFFICE tionfo'f Delaware invention relates to heat exchangers and more particularly to that type of heat exchangeror coolers have particular advantage where high heat transfer efficiency is required and especially where the volumes of the heat-exchanging fluids are small and their temperature difference is relatively great. It is also known that the use of a finned or extended surface tube in such a heat exchanger materially increases the heat exchange efficienoy where the fluids undergoing heat exchange have relatively different heat transfer characteristics. For example, in the transfer of heat energy from a gas or a vapor such as condensing steam to a liquid such as oil, a substantial increase in heat transfer can be obtained by the use of a finned tube. It will be apparent, however, that there is a limit to the extended surface possible for any tube.

The principal object of our invention is to increase the heat exchange efllciency of a finned tube double pipe heat exchanger whereby a greater heat transfer rate can be obtained per unit tube length.

Another object of our invention is to provide a standardized finned tube double pipe heat exchanger, in which the outer tube may be a standard size pipe and the fin on the inner tube is adapted to closely flt the outer tube and in which the ends of the tubes are provided with simple caps for headers.

A more specific object of our invention is to provide a helically finned inner tube for a double pipe heat exchanger, the fin of which engages in a helically threaded recess in the outer tube in such a manner that the inner tube may be screwed into position in the outer tube to present a substantially liquid-tight construction that prevents the by-passing of the external fluid between its inlet and outlet.

Further objects and advantages of our invention will appear from the following description of preferred forms of embodiment thereof taken in connection with the attached drawing, in which:

Fig. 1 is a substantially central vertical section with parts in elevation of a double pipe heat exchanger;

Fig. 2 is a view similar to Fig. 1 showing a slightly modified form of construction;

Fig. 3 is a vertical section with parts in elevanecessity the same.

continuous path of flow is thus provided Harold M. Graham, Bronxville, and Frederick D. Berkeley, Chappaqua, N. Y., assignors to The I Lummus' Company, New York,.N. Y., a corporan iianb October 31,1 4 serial No. 417,290 i 2Claims. (c1. 257--246) which may be a standard piece of pipe cut to a predetermined length. According to our invention, the internal surface of this outer tube is provided with av helical channel or recess l2 extending substantially throughout the length of.

the tube. This channel has a depth and a width that adapt it to receive the edge portion of the helical fin l4 formed on the external surface of the inner tubular member H5.

The inner tubular member l6 may be conveniently made of relatively thin metal, and the helical fin It may be formed thereon by any suit able process. Preferably, however, the construction is such that fin I4 is integral with tube IS. The fin may also desirably have a triangular cross section as shown; for in this manner a more efiicient conduction of heat through the extended surface is assured. Fin [4 is continuous throughout its entire length but does not extend to the ends of the inner tube [6. The terminal portions of the external surface of tube [6 are thus free of a finned surface for a purpose more fully explained hereinafter.

As already indicated, in this form of embodiment the external diameter of fin I4 is slightly greater than the internal diameter of the outer tube ID. It will be apparent that the pitch of the channel 12 and the pitch of the fin I4 are of Accordingly, when the unit is assembled, helical chamber H is formed by the fin and the external surface of the inner tube and the internal surface of the outer tube. A long for the external fluid, which path may be many times the length of the path of flow of the internal fluid.

This double pipe unit may be conveniently assembled by inserting the inner tubular member l6 into the outer. tubular member l0 so that the helical fin I4 engages the helical channel l2 and then rotating tube l6 until it is in the desired position. -Each end l8 of the inner tubular member, being free of fin l4, may then be suitably expanded into contact with the internal surface of the outer tubular member In to form a relatively tight joint to prevent leakage of the external fluid. For this purpose the length of both of the tubular members is desirably made the same. Expansion of ends I! of the inner tube It also serves to lock the inner tube in the desired position with respect to the outer tube III.

Helical chamber I1 is made fluid-tight by reason of the engagement of fln Il in the helical channel I2. By-passing of the external fluid between inlet nozzle 20 and outlet nozzle 2I is substantially and, in most cases, entirely prevented since there is little, if any, possibility for such fluid to flow across the outer edge of fin Il. Accordingly, all of the external fluid must follow the helical path defined by chamber H with its relatively great heat transfer surface. I

Suitable headers such as internally screwthreaded caps 22 and 23 are adapted to be secured to the ends of external tubular member III, which is suitably threaded as shown. These caps are provided with nozzles 24 and 25, respectively, for the introduction of the internal fluid into inner tube It and the discharge of such fiuid therefrom. By virtue of the expanded terminal portions of the inner tubular member I6, the two heat exchanging fluids can be maintained in indirect contact. The two fluids may fiow in countercurrent relation as shown or in concurrent relation if desired.

This form of embodiment of our invention may be readily assembled for use and quickly disassembled for inspection, cleaning, and the like. It has a high heat transfer efliciency, inter alia, because of the prevention of by-passing of the external fluid across the fin Il and becauseof the increased turbulence of flow of the external fluid. A plurality of double pipe units of this type may be combined into one larger unit where such an assembly is required or practicable as when a substantial amount of heating or cooling must be done. It will be appreciated that a single unit may also be made of suflicient length to provide single-pass operation. The apparatus is particularly well adapted for the heating of fuel oil or the like as by steam.

A slightly modified form of embodiment of our invention, which is somewhat cheaper to construct than that shown in Fig. l, is shown in Fig. 2. In this case the outer tubular member Ilia is similarly provided with inlet and outlet nozzles 20 and 2I and is also similarly provided with screw-threaded caps 22 and 23 for the passage of the external fiuid through the inner tubular member I6a as by nozzles 2l and 25. Inner tubular .member IGa. is likewise similarly provided with a continuous helical fin indicated at Ila. In this embodiment, however, the external diameter of fin Ila is substantially the same as the internal diameter of the outer tubular member Ilia. The relation between the two diameters is preferably such that fin Ila is in contact with the internal surface of outer tubular member Ilia whereby substantially no by-passing of the external fluid across fin Ila occurs as such fiuid flows through helical path I'm. The elimination of the helical channel or recess in the outer tubular member Ilia permits the direct insertion and removal of the inner tubular member I6a.

In this construction each end IBa of the inner tubular member Ilia. may be sealed with respect to the outer tubular member Ilia by expansion of each terminal portion of the inner tubular member to force it into a circular groove IIlb located in the corresponding terminal portion of the internal surface of the outer tubular member in accordance with the customary manner of expanding tubes in tube sheets in condensers and the like. This particular construction in some cases may be more effective than the mere expansion as shown in Fig. 1. Especially where the heat exchanging fluids are such that mutual contamination should be avoided, the method of expansion shown in Fig. 2 is to be preferred.

A still further modification of our invention is shown in Figs. 3 and 4. In thistype of double pipe heat exchanger, we have provided a similar form of outer tubular member 30 having the inlet nozzle 20. The inner tubular member 32 is provided with a fin 3l, the external diameter of which is substantially the same as the internal diameter of the outer tubular member 30 as was the case with the unit shown in Fig. 2. Each end 35 of the inner tubular member 32 may be expanded in the manner as shown in Fig. 1 to make a tight joint with the outer tubular member 30. A screw-threaded cap (not shown) may also be grovided for each end of the outer tubular mem- It will be noted that, in this construction, helical fin 3l is discontinuous and that it is divided into a plurality of discontinuous sections such as 34a, 34b, 34c, and 3ld. These discontinuous sections are preferably alternately arranged so that the helical 'motion imparted to the flow of the external fluid by the helical fin will not betoo greatly affected. In the particular construction shown the discontinuous sections are alternately and diametrically opposed. It will be understood, however, that the arrangement of these discontinuous sections may be any desired such as the positioning of the discontinuous portion of a subsequent section at an angle of from the discontinuous portion of the preceding section and that the discontinuous portions of all the sections may be in a straight line if desired.

The advantage of such construction is that a greater amount of liquid can be passed through the helical chamber l0 between the fin and the two tubular members while the features of the heat exchange units shown in Figs. 1 and 2 are retained to a substantial extent. The helical shape of the fin gives the external fluid a substantially helical path of flow, but at the same time its discontinuous construction permits a portion of such fluid to flow in a straight'path. Accordingly, an average rotating velocity is obtained while the pressure drop which the external fluid suifers during its fiow is not so great because of the partial by-passing thereof through each discontinuous section of-fin 34. Since the discontinuous portions of the several discontinuous sections of fin 34 may be alternately arranged in any particular manner along the length of the inner tubular element, there is an alternately longitudinal'a well as a helical motion imparted to the external fluid in its extended path of flow. A relatively high heat transfer efllciency is maintained because of the thorough mixing accomplished and the improved surface contact obtained. It will be apparent that this discontinuous feature may be applied to a double pipe heat exchanger of the type shown in Fig. l.

Foreconomy of space, lightness of weight, low cost, and high eiiiciency. an exchanger of this character is especially satisfactory; and material benefits result from its use on installations within its eilective range. It will be appreciated that the factors of length and of duplication of a unit are primarily dependent upon the desired or requisite amount of" heat transfer and the nature of the heat transfer mediums. It will also be appreciated that the helical fln constructions shown in Figs. 1, 2, and 3 may be used in a double pipe heat exchanger in which the terminal portions of the inner tubular member are secured in relation to the outer tubular member in any other suitable manner.

While we haveshown and described preferred forms of embodiment of our invention, we are aware that other modifications may be made thereto; and we, therefore, desire a broad interpretation of our invention within the scope and spirit of the description herein and of the claims appended hereinafter.

We claim:

1. A double-pipe heat exchanger comprising an outer tube of substantially uniform internal diameter and having a fluid inlet and a fluid outlet spaced therealong, an inner tube having its terminal portions expanded into sealing contact with the internal surface of the outer tube to form fluid-tight joints therewith and to form an outwardly flaring inlet at one end of said inner tube and an outwardly flaring outlet at the other end thereof, means for passing a fluid through the inner tube, and a helical fln on the external surface of the inner tube and terminating short of the ends of the latter, a number of successive convolutions of said fln having gaps in alignment longitudinally of said tubes, and a following number of successive convolutions of the fin having uninterrupted portions in opposition to said gaps and also having gaps spaced around the inner tube out of alignment with said first gaps, for the purpose set forth.

2. A double-pipe heat exchanger comprising an outer tube, an inner tube extending longitudinally within said outer tube, means for passing a fluid through said inner tube, means for passing a fluid through the space between said tubes for indirect heat exchange with the fluid in the inner tube, and a helical fin within the space between said tubes and extending therealong and around the exterior of the inner tube, a number of successive convolutions of said fin being formed to provide openings therethrough at points in alignment along the tubes and a following num ber of successive convolutions of said fin having uninterrupted portions in opposition to said openings and being formed to provide openings therethrough spaced around the inner tube out of alignment with said first openings, for the P p se set forth.

HAROLD M. GRAHAM. FREDERICK D. BERKELEY. 

